Picophytoplankton dynamics in the equatorial Pacific: Growth and grazing rates from cytometric counts

During a 7 day time series in the central equatorial Pacific (0 degrees, 15 0 degrees W, October 1994) flow cytometry measurements were performed four times per day throughout the surface layer. Cell abundance of the major alg al groups, Prochlorococcus, picoeukaryotes, and Synechococcus, exhibited a well-marked diel rhythm within the mixed layer (50-60 m deep) whereas the s ignal became unclear below. Cell numbers were minimum at the midday or dusk stations and maximum in the middle of the night. The amplitude of the diel variations in the mixed layer, as observed, was of the order of 40% of the daily minimum and varied significantly during the time series. For each ce ll group the abundance variations implied that each day, cell division was rather tightly synchronized and that grazing was efficiently competing grow th. Assuming that abundance variations were only due to these two processes , a simple model was designed to estimate their rates. The division rates a veraged 0.53 (+/-0.18) d(-1) for Prochlorococcus, 0.42 (+/-0.13) d(-1) for the picoeukaryotes, and 0.56 (+/-0.21) d(-1) for Synechococcus in the mixed layer and decreased rapidly below. The rates varied significantly along th e time series for all groups whereas growth and grazing closely balanced at the day scale. The estimates compared well with those obtained using vario us methods during the time series and previously in the equatorial Pacific. Primary production was tentatively predicted from the growth rates. Prochl orococcus, the picoeukaryotes, and Synechococcus contributed 57%, 33%, and 10% of the picoplankton total, and the predictions were consistent with the C-14 measurements during the time series.